As a working hypothesis a model of protein folding has been developed. The three-dimensional screened precession data of crystals of Nuclease-T (an enzymically active, non-covalent complex of two fragments of staphylococcal nuclease, residues 6-48 and 49-149) bound with deoxythymidine-3'-5'-diphosphate (pdTp) and Ca2 ion have been collected at 2-4 degrees C to beyond 2.7 A (see the report of the previous year). Phases were calculated for the observed data using both the coordinates of the intact enzyme bound with pdTp and Ca2 ion, as reported by M.J. Legg (1977, Ph.D. dissertation, Texas A & M Univ.) and the same model partially refined against the nuclease-T data (by collaboration with G.H. Cohen, LMB, NIAMDD). The nuclease-T molecule with pdTp and Ca2 ion can be fully traced in these maps and is highly similar to the intact nuclease model. Cytochrome c synthetase catalyzing the formation of thioether bonds between heme and yeast apocytochrome c is found in a yeast mitochondrial fraction. The activity of this enzyme markedly increases by addition of NADP(H) or NAD(H) (the reduced form being more effective than the oxidized) and a heat stable postmitochondrial factor. The yeast enzyme can catalyze also the covalent bonding of heme to horse apocytochrome c.